1. Field of the Invention
The present invention relates generally to a stent with improved flexibility. More specifically, the present invention relates to a welded stent having increased flexibility at the welded connections.
2. Description of Related Art
A stent is a prosthesis that is inserted into a body lumen and used, for example, for treating stenoses, strictures, and/or aneurysms therein. In the event of a stenosed vessel, a stent may be used to prop open the vessel after an angioplasty procedure. Once opened, the stent forms to the inner wall of the vessel, remains in place, and may help prevent restenosis. Additionally, in the event of an aneurysm or weakened vessel wall, stents may be used to provide support to, and reinforce the vessel wall.
To perform such functions, stents in the past have included many different structures. For example, previously disclosed stents include coiled stainless steel springs, helical wound springs, and generally serpentine configurations with continuous waves of generally sinusoidal character. Some of these stents self deploy when placed in the vessel, whereby stent expansion is primarily achieved by removing a restraint mechanism holding the stent in a constricted state. Other types of stents rely on alternate means to deploy, for example, use of a balloon catheter system, whereby balloon dilation expands and deploys the stent.
One of the major complications associated with using stents has been thrombosis. This complication is caused by clotting in the vicinity of the stent and is associated with high morbidity and mortality. It has been shown that the better the stent apposition against the vessel wall and the larger the lumen, the less likely that this complication will occur. A further complication is restenosis, which is caused by tissue proliferation around the angioplasty site. To minimize the potential for restenosis, the stent should cover the lesion and not leave any significant gaps in which restenosis may occur. The stent should also adhere to the inner wall of the vessel as much as possible.
Accordingly, when a stent deploys in a restricted vessel, adequate radial strength is required to overcome the strictures and ensure apposition of the stent to the vessel wall. Radial strength is a force produced by the stent acting at all points on the vessel wall in an outwardly direction perpendicular to the vessel wall. Stents are designed with circumferential rings to provide most of the radial strength needed to overcome radial forces pushing inwardly against the stent as the stent expands.
Many stents also include longitudinal links that primarily act to attach longitudinally adjacent circumferential rings, but also add radial strength and stent stability. Once the stent is fully deployed, in addition to providing adequate radial strength, the stent must provide adequate vessel wall coverage, hereinafter referred to as scaffolding affect. Scaffolding affect is defined as the amount of area of the vessel wall covered by the stent, once the stent is fully deployed. The circumferential rings and longitudinal links connecting the circumferential rings have traditionally provided the needed scaffolding affect. Other stents include welded connections between longitudinally adjacent circumferential rings.
Further, to meet the demands of adequate radial strength and scaffolding affect, conventional stents have been designed with circumferential rings manufactured with adequate ring width, which were then continuously connected at each peak and valley or trough by longitudinal links. However, such conventional stents suffer from predilation stent longitudinal rigidity. Predilation or crimped stent longitudinal rigidity is a resistance to movement and decreased flexibility of the stent along the stent's longitudinal axis. Accordingly, predilation longitudinal stent rigidity makes it much harder and oftentimes even impossible to thread the stent through long tortuous vessels and past constrictions and lesions.
Past attempts have been made to overcome predilation stent longitudinal rigidity. Such attempts have included designs with decreased ring width, often referred to as decreased wire gauge, which resulted in increased longitudinal flexibility but decreased radial strength. These conventional designs have resulted in inadequate stent apposition and/or inadequate vessel wall support. Additionally, past attempts to increase longitudinal flexibility have included designs where longitudinal links are not attached to each peak and valley of the circumferential ring. Thus, only some of the peaks and valleys of adjacent circumferential rings are connected by longitudinal links. This increases longitudinal flexibility but decreases the scaffolding affect of the stent. The decreased scaffolding affect creates areas where the vessel wall is not adequately covered by the stent, which may lead to thrombosis and/or restenosis.
Additionally, in order to meet the requirements of drug eluting stents, conventional stent substrates have been designed with circumferential elements manufactured with adequate ring/strut/apex width, which were then continuously connected at each peak and valley by longitudinal links. However, such conventional stents may suffer from abrasion or damage due to adjacent apexes (i.e., peaks and valleys) interacting during crimping and tracking, which may be caused by the close proximity of adjacent apexes coming into contact with one another due to links or weld not providing adequate clearance. This interaction may cause abrasion or damage during the coating of the stent with a drug and/or polymer or during tracking of the stent through the anatomy.
Accordingly, there arises the need for a stent, which provides adequate radial strength, scaffolding affect, with increased apex spacing and longitudinal flexibility. It is among the objects of the present invention to provide a stent that overcomes the foregoing shortcomings and meets the needs discussed above.